In conventional injection molding methods and apparatus, a material, such as plastic, is supplied to a barrel where it is heated to molten form by electric heaters or the like. The molten material is than forced under pressure by a reciprocating screw through a nozzle. The molten material passes from the nozzle through a manifold, to at least one bushing, and then into a mold cavity to form a part.
In order to injection mold consistent parts, it is important to keep the molten material heated at a constant temperature throughout the injection molding process. This is particularly true where engineering-type plastic materials are utilized which have a relatively small working temperature range.
In order to insure that the molten material remains heated to the proper temperature or within the requisite temperature range, it is important to regulate the temperature of the material at the various locations through which the molten material passes, including the manifold and bushings. If the temperature of the material fluctuates significantly, the injection molding process cannot produce consistent high quality molded parts. This will increase the number of rejected parts and waste materials.
If the molten material is heated to a higher or lower temperature than desired, its flow characteristics can change and more or less molten material may be provided to the part than required. The quality and finish of the final molded part also will be affected. Temperature fluctuations frequently occur as a result of variations in the line voltage being used to supply power to the temperature controller which in turn is used to control the heat applied to the desired areas of the molding system, including the manifold and the bushings.
In current injection molding processes, the temperatures at locations in the various components of the system are monitored by thermocouples. The measured temperature from each thermocouple is displayed on a temperature controller. If any changes in the temperature at those locations are detected, they are reflected by the thermocouples. The thermocouples send the data to the temperature controller which is set-up to adjust and correct the temperatures of the components as needed. However, if there is a delay from the time the temperature first changes to the time the thermocouple detects the temperature change and the controller can react to the change by increasing or decreasing the voltage to correct for the temperature variances. This time delay can affect the quality of the part.
Systems for monitoring a line supplying power and systems for correcting variances in the supplied power are known in the art. Examples of such systems are disclosed in U.S. Pat. Nos. 4,356,440, 4,554,502, and 4,672,298. However, these prior art systems relate to power factor corrections for voltage being applied to inductive loads, such as motors and not to resistive loads such as electric heaters and the like. Inductive loads respond differently to line voltage fluxations than do resistive loads. Thus, these systems as they relate to inductive loads do not assist in the problems of line voltage variances as they affect resistive loads.